1. Field
This invention relates to detecting flaws in magnetizable test objects by measuring the magnetic flux leakage caused by such flaws. The invention particularly relates to inspection apparatus which may be referred to as inspection pigs and which are adapted to be run through and to inspect buried pipelines which transmit petroleum products and other fluids. These inspection pigs are designed to provide information about the pipeline's deterioration (such as pits and cracks caused by corrosion, stress or other causes) without the relatively great cost of unearthing the pipeline. This information is particularly important because buried pipeline is expensive and because a pipeline explosion can destroy adjacent property and take human lives.
2. Prior Art
A used throughout this description and in the claims, the terms "magnetize" and "magnetizing" and related terms refer to the concept of introducing a magnetic field or magnetic flux into a region or object. The inspection of a magnetizable test object by magnetizing the object and measuring the magnetic flux leakage adjacent the object and caused by flaws in the object is relatively old in the art. For example, U.S. Pat. No. 1,867,685 (1932) to Sperry discloses such a device for railway rails. Further, inspection pigs for inspecting various conditions in pipelines have been known for over ten years. See, for example, U.S. Pat. No. 2,782,370 (1957) to Ver Nooy and U.S. Pat. No. 3,064,127 (1962) to Green et al.
Inspection apparatus generally referred to as inspection pigs typically are propelled through a pipeline by the pressure of the fluids therein and thus may go through a pipeline without substantially disrupting its operation. These pigs frequently comprise one or more supports, annular cups attached to the supports and engaging the inner wall of the pipeline, magnets mounted on a support for magnetizing the pipeline, detectors mounted adjacent the magnets for measuring the flux leakage, and recorders, such as strip charts and pens, mounted on a support for recording the flux leakage measurements.
In the known prior art, several basic sensing means or detectors have been used to sense flux leakage. Moving a current-conducting wire through a magnetic field will induce in the wire electric current proportional to the speed of movement and the strength of the magnetic field. This type of detector has broad application but has the obvious disadvantages that the wire must be moving with respect to the magnetic field to produce a reading and that the amplitude of the reading will be proportional to the velocity of the wire. Detectors such as Hall elements and magnetometers do not depend on movement with respect to the magnetic field, but these devices are relatively complex and expensive, require relatively complex and expensive complimentary electronic circuitry and must be driven by a power source which adds weight and expense to the inspection device.
Magnetic diodes are relatively new semi-conductor devices which change their internal electric resistance as a function of an external magnetic field. They measure magnetic flux independently of their velocity with respect to the flux and are from 10 to 100 times more sensitive than Hall elements. Magnetic diodes are relatively simple and inexpensive and require relatively small amounts of power; however, they are quite small, typically having a length of no more than 1/4 inch. Magnetic diodes may be connected in pairs to form twin diodes which function independently of temperature. Magnetic diodes are known to have a variety of applications suitable to their small size, such as generating pulses as disclosed in U.S. Pat. No. 3,689,836 (1972) to Snyder, providing a commutating device for a brushless direct current motor as disclosed in U.S. Pat. No. 3,688,172 (1972) to Sieber et al and detecting flux leakage from relatively small areas of movable test objects which are translated and rotated with respect to the diodes, as disclosed in U.S. Pat. No. 3,670,239 (1972) to Shiraiwa et al. However, because of their small size, magnetic diodes have no obvious application with test objects such as pipelines that are relatively immovable and relatively large as compared to the usual one-quarter inch size of said diodes.
In the prior art, problems have been encountered in maintaining the magnets and detectors in sufficiently close proximity to the test object to magnetize the test object and to detect magnetic flux leakage therefrom caused by defects therein. These problems are particularly acute with pipelines because the magnets and detectors should be maintained close to the pipeline wall, preferably at a constant distance therefrom, but should also be capable of moving radially inwardly and outwardly responsive to variations in the internal diameter of the pipeline without damaging the pipeline to accommodate decreases in the diameter of the pipeline caused by dents, welds and other obstructions. In the prior art, various spring mechanisms have been used to urge the magnets and detectors toward the pipeline, but these mechanisms have been unduly heavy and bulky, are subject to failure, require maintenance and are relatively expensive.
In the prior art, problems have been encountered in determining the location of pipeline flaws detected by a moving flux leakage inspection apparatus or pig, because the pig seldom travels through the pipeline at an even or known speed due primarily to changes in the terrain and pressure within the pipeline. Odometers attached to the pig in engagement with the interior wall of the pipeline can supply valuable information, but have been inaccurate because of slippage. U.S. Pat. No. 3,064,127 (1962) to Green et al discloses the use of radioactive markers placed at selected locations along a pipeline and U.S. Pat. No. 3,116,457 (1963) to Schmidt discloses the use of coil markers placed along a pipeline to influence eddy currents, but such markers clearly are not useful for inspection pigs depending on flux leakage detection. Magnetic markers placed at selected locations along the exterior of the pipeline to influence flux therein have many advantages, but in the prior art their signals were easily confused with signals caused by stopples, nipples and other hardware attached to the line and thus separate equipment has been required for detecting marker signals, thereby adding to the weight and cost of the pig.